A parabolic reflector pointed at a clear
night sky will tend to bring the
temperature of a body at its focal point
into equilibrium with the cold vastness of
space, at approximately 3 kelvin. You
can easily prove this at home with some
reflectivey stuff and a bucket of water.
You may
even get it to freeze. [see link
1]

Cloudless desert nights can be icy icy
cold, which anyone who's been to central
Australia can testify.

I propose a passive home cooling and
refrigeration system for hot, cloudless
environments i.e. deserts.

A rooftop parabolic or compound
parabolic [link 2] trough system, as
known from solar thermal heating
systems, is exposed to the night sky,
pointing straight up, and used as a
thermal emitter (radiator) rather than a
thermal concentrator.

Through black tubes at the foci runs a
fluid with a high thermal capacity and a
low freezing point, perhaps a salt/water/
antifreeze mixture. These tubes
converge into a manifold that runs to a
storage tank, situated lower than the
emitter plates. This in turn is connected
via controlled valves to "radiators" in the
rooms requiring cooling during the day,
and by a separate (and separately
regulated) line to a cool-room or
refrigerator.

I say "radiators" because these will look
much like (and may even be) the
baseboard radiators seen in buildings
with hot-water central heating, but will in
fact be acting as thermal collectors
("emitting" cold) and will need to be high
in the room where the hot air gathers,
rather than low.

The heat gathered here causes the
coolant to thermosiphon [3] through the
system.

During the day, as desired by the
inhabitants or by thermostat and
solenoid, the valves between the cold
storage tank and the room collectors are
opened. The heated fluid from the
rooms rises, taking the heat with it (or,
to put it another way, the chilled coolant
sinks to the rooms below, and absorbs
heat) tending to equalise the
temperatures of the room and the tank
(or to put it another way, keeping you
and your food, medicines, morgue
inhabitants, etc cool in summer).

At night, the valves between the tank and
rooftop radiators are opened, and
thermosiphoning begins anew. The
slightly warmed coolant rises into the
black tubes. From here, the excess heat
is dumped into space by radiation.

Insulated shutters come down over the
radiators during the day to stop the
coolant from boiling. Should the coolant
freeze solid in the tubes, the shutters can
be opened awhile.

Alternatively, one could under-fill and
under-pressure the system and create a
phase change heat pump [4] or heatpipe
[5], where the fluid boils at the hottest
point and re-condenses at the emitter.
This would allow much more rapid heat
transfer at the expense of more critical
materials tolerances and seals.

Attainable temperatures will depend
mainly on the freezing and boiling points
of your chosen refrigerant. Liquid
nitrogen should work nicely :) The size
of your storage tank will depend on the
area you wish to keep cool, local ambient
temperatures, and the ratio between
cloudy and clear nights in your location
(reserve capacity).

Using more common fluids or water, this
system could be built with standard
plumbing and A/C parts, built and
certified by any qualified refrigeration
engineer, or home-brewed by hobbyists,
survivalists, low-impacters and villagers.
It is as applicable to skyscrapers in
Houston as is is to medical clinics in
Africa. It consumes no power and has no
moving parts other than the valves.

It mitigates global warming two ways:
indirectly, by displacing power-hungry
active refrigeration, and directly, by
heatsinking the Earth.

Beaming heat into space could cut airconditioning costshttps://cosmosmagaz...rconditioning-costsA team led by Stanfords Eli Goldstein report in the journal Nature Energy on the creation of fluid cooling panels that harness radiative sky cooling to cool fluids below the air temperature with zero evaporative losses, and use almost no electricity. The panel is remarkably simple, using acrylic walls, polystyrene cover and insulation, and a plate heat exchanger through which water flows. By radiating and reflecting infrared radiation high into the atmosphere and beyond into outer space, the system effectively uses them as remote heat sinks. [BunsenHoneydew, Oct 30 2017]

Radiant heat is infra-red light. Normally, a body's radiant loss is more-or-less matched by the radiant heat returned from items around it, and equilibrium is attained at what you or I would consider normal-ish temperatures.

But if you put a radiant barrier (and associated nonconductive insulation [6]) between the radiator pipes and the Earth, your house, etc, and focus it such that the only way is up, then the radiant heat is sent out into space (barring that blocked by the greenhouse effect - I think the lack of moisture in a desert helps here), and only the cosmic background radiation at 3 kelvin returns.

Which gives me a new name :)

If you doubt this, I suggest you try sleeping naked on top of a space blanket on a starry night in the middle of a large flat desert. You can literally feel the universe sucking the heat from your body.

Parabolic and straight up is the shortest path through the atmosphere, and minimises the greenhouse trapping, but as long as your reflector totally blocks line-of-sight to the Earth and associated objects, it should be partly effective.

A huge subterranean cistern could provide baseload for the summer months, collected over winter. This would require either active pumping, or building on a slope with a cavern uphill from the house. An insulated concrete water tank is another option.

/reads back/ Geez, I'm turning into Vernon, except with line breaks.

A self-contained refrigeration unit for remote communities could be constructed from a standard insulated food transport shipping container with a roof-mounted radiator. The storage tank would be unnecessary; the cool air and food inside the container would be the cold store. Temperatures would vary, but significant cooling would still be attained. An airlock door system would help.

I am reminded of something I once saw on tv - A scientific experiment from the stupid ages in which a reflective dish was made and filled with ice, in the hope that the cold would be reflected to a point.

Sadly it was too far back for me to remember whether it was serious or a comedy thing - a long search on t'internet reveals nothing.

//are you really going to get any more refrigeration than the ambient temperature//

Yes. See the first link, and the sixth.

Heat transfer to/from ambient air is mostly by conduction and convection. You insulate against that. All you have left is radiant transfer, and you point that into the void.

It's not energy from daily temp differential - it's energy *dumping* from here to the chilly, diffuse embers of the big bang. This is how space probes, ships and stations dump excess heat, except they have no need to focus the effect.

Read the heat transfer and insulation link. Then read link [8]. Make sure you get the difference between radiation, conduction and convection.

Link [8] probably sums it up best.

Of course, you're never going to get close to 3k, for the reason [DrC] gave in the first anno, and because of the atmosphere. But the greenhouse gases only absorb a proportion of the IR, and of that, half is re-radiated into space, and most of the rest misses the path back to the dish. Even if it's 95% inefficient, we're talking about a nearly 300k temperature differential to work with.

When conduction and convection are insulated against, black body radiation becomes the main source of heat transfer, assuming it is not also insulated against.

//when the sun rises there will be shit load of radiation coming from all directions, so shirley this a day/night energy differential idea ?//

No, it's an Earth/space energy differential idea. If you're still doubtful, grab two thermometers, a cardboard box, a space blanket and a black bucket and repeat the experiment given in the first link.

But your question brings up an interesting point. I suspect that if the focus was tight enough, ie at the bottom of a deep, steep sided parabolic well, this would still work in the daytime. After all, space is still as cold as it was last night.

In an ideal system, and if you wanted to achieve scarily low temperatures, you would transmogrify the wavelength of the emitted radiation to outside the greenhouse-attenuated range. I'm a bit lost for a mechanism here - something like the coating on fluorescent tubes, but tuned for IR, perhaps. /edit/ Or lazerrrrrrs

[fridge duck] - sounds like it would be more effective to surround the object with a sphere of ice, so the object can dump heat equally in all directions. "Cold" is not a form of radiant energy, although back in the stupid ages people thought it was a fluid called phlogiston.

It's close, but it doesn't insulate the radiator from ambient air. It's closer to a day/night ambient temp system than an Earth/space system, although it has an element of that. The formulas under "Description of..." look useful, and when I've had some sleep I'll have a closer look at them.

//But this may work if you could build an enormous funnel of void ... bypassing the Earth's atmosphere.//: Lazerrrs man, laaaazeeerrrrs. And my next album is going to be called "Enormous Funnel of Void"

The key phrase in [baconbrain]'s link seems to be "effective sky temperature". This sums the CBE with the radiant heat from the atmosphere, far as I can see.

//[excised]//: I apologise for the tone [baconbrain]. That applies equally to my opinion of course. Your argument does have a point, and I will have to check in with the universe and get back to you all anon.

Meanwhile, it occurs to me that I've learned an enormous amount about physics, thermodynamics, materials science, quantum mechanics, electronics, politics, economics and so on from posting, annotating, arguing and researching here at the HB. It's like a Steiner school for mad engineers.

That and sleep deprivation is great for halfbaking, but not so great for maths.

//A huge subterranean cistern could provide baseload for the summer months, collected over winter. This would require either active pumping, or building on a slope with a cavern uphill from the house. An insulated concrete water tank is another option.//

Wouldn't it be nice if we could store the heat from summer and use it in the winter to heat the house, and "store" the "cold" (manner of speaking) from winter and use it in summer to cool the house? Unfortunately, I don't think we have something with enough heat capacity (mass) that can be stored with a house. You need an small ocean for that.

I like your idea. I have no idea if it could work. Can't you build a small model and try it out? Even nicer if the reflector could be used for both cooling and heating.

The cosmic background radiation "has a thermal 2.725 kelvin black body spectrum" (from Wikipedia), but the effective temperature even with an //enormous funnel of void// (I've got to work that into a conversation somehow) would be higher due to stars and such. Effective sky temperature appears to be only 10 to 20 degrees colder than ambient, which is still cold enough to be useful. You aren't going to make liquid nitrogen with this, but (+).

//enough heat capacity// There's a (baked) method where external insulation is extended a few metres into the ground around a building, effectively giving hundreds of cubic metres of thermal mass rather cheaply, as dirt.

This is one of these ideas that sounds insane when proposed, but I suspect will sound like old hat in fifty years when concepts like this have been refined. Give Beaker a holiday when the orders start coming in.

I'm pretty sure this would work as described; my only concern is the cost. The thermal storage unit is the one that is most expensive, I think, because it must be both large and very well insulated. I would love to hear comments on how one makes such a thing without breaking the bank. I don't see any links or significant discussion on that part of the system.

The parabolic idea is nonsense. All of the sky visible to the body is irradiating it, and focusing the heat radiation of the body back into the sky does nothing at all. So this is no better than putting the body in a reflective, insulated bucket. In other words, highly baked.

//Effective sky temperature appears to be only 10 to
20 degrees colder than ambient, which is still cold
enough to be useful.// That's what I love about
halfbaking: you start with something completely
wacky, "no you can't" "yes you can" "no y- hey what
about this?" "oooooo that might work"

The system in [baconbrain]'s link is definitely cheap
and simple to set up, and if the effective difference is
only 10 to 20 degrees (was that C or F [spidermother]?)
then chuck all the complicated plumbing.

If I can dig up a fluorophore that can give me two
greenhouse-invisible photons for every near-IR one,
this might still be worth pursuing.

//Wouldn't it be nice if we could store the heat from
summer ... I don't think we have something with
enough heat capacity // I've heard or read somewhere
of people building basements full of loose granite
rocks (bluestone) and pumping solar heated air
through it. That should have a fair heat capacity.

//Even nicer if the reflector could be used for both
cooling and heating.// Yes I thought about that too -
but my idea was already turning into the Illiad and I
needed a break. Install a second, uphill storage
tank/heat exchanger, tilt the panel to a sun-friendly
angle and open a second set of valves.

Another variation would be shutters in the insulation
around the radiator that allow cold night air in, then
close when you want to take advantage of the infra-
red dumping for the last few degrees of cooling.

I like the idea, but can we just gently go through a thought experiment for a moment pls?

Two metal objects float in the void - nothing else exists anywhere, except for these two objects.

They are made of some special alloy that allows them to change shape (parabolic etc), but lets just imagine them being cylindrical for the moment.

They're also at some temperature - whatever you like, +50C, +50K, whetever. Only they're at slightly different temperatures (say 100K different).

The question, is the amount of radiation they emit a function of anything other than their temperature?

What if you surround with some insulating substance? Or a shiny, reflective material?

Whatever you do, the radiation they emit remains the same (it might be reabsorbed in the case of the reflective materials, keeping the bar warmer for longer). Their IR brightness remains a function of their temperature relative to Absolute Zero, not relative to their surroundings, their neighbours, or the amount of ambient radiation present in the ether.

What's surprising is that the ONLY way for the objects in this situation to lose energy, is through radiation.

That's not to say this idea doesn't work, just the cooling properties have nothing to do with the temperature differnetial between the earth and space. Yes, there is energy transfer, yes the energy is being beamed out into the void - but you could beam it at anything you want, and you should recieve the same levels of local cooling.

Perhaps, with enough of these devices, all beaming their extra energy to a single, focal point, you might be able to raise the temperature of something by a few degrees - but no more or less energy will be transferred than if you aimed it straight up at the night (or day) sky.

Mono, the answers are: 1. It doesn't, no more than water knows to go downhill. 2. Don't worry about it. All bodies radiate all the time, depending on the emissivity and the fourth power of temperature. 3. When something gets in the way, then it's a three body problem, not yours.

//there is still something tangible that 'attracts' it downhill//Nothing attracts heat, but all things radiate, so it's a matter of balance--heat being absorbed minus heat being radiated. The sun warms the earth and the earth re-radiates that heat away. If it didn't, we'd quickly cook. That space is *cold* is just a convenient fiction. It seems cold only because it isn't a source of photons (except for starlight and the remnant energy from the big bang). Which is why the parabolic reflector is nonsense. To cool the body it is only necessary to block incoming photons while allowing outgoing photons to escape to the night sky (which has a low effective temperature). It doesn't matter that they escape in a specific direction, so focusing them is pointless.

Another point Bunsen misses is that the body won't get any colder than the reflector, so that has to be well insulated. Conduction and convection are also transmitting heat, and far more effectively than radiation at ambient temperature. If the reflector is warmed by convection from the back side, it will radiate from the front side, warming the body. But the reflector is actually unnecessary. All you need is a block of Styrofoam with a hollow in the top. Fill it with water and expose it to the desert sky on a night when there's no breeze, making sure that the surface of the water can see nothing but sky. The water will get cold, sure, but as a radiator, the set-up is inefficient, as the equivalent patch of desert sand will radiate much more heat to the sky.

I'm really impressed. I really didn't believe the effect would be noticeable, but it appears to be actually useful. Blimey. (unless the internet is lying to me... but nah, it would never do that, would it?)

I have never been able to find a link, but I have a book showing a diagram of an ice wall.

"Ice walls, another ancient method of passive cooling, had been used in the deserts of the Middle East & In the East Indies up to the early part of this century (book published 1978) to make ice during nonfreezing weather.
The long east- & west-oriented earth walls prevent direct solar gain on shallow troughs of water located on the shaded north side. At night, the water radiates long-wave heat energy to deep space. The ice wall allows air stratification in the wind shadow. Insulation of the trough from the earth isolates the water from the ground temperature.
Buttresses, perpendicular to the wall, structurally reinforce it & prevent solar gain from the east & west, helping to still air movement. Two or more parallel ice walls also aid air stratification between them. Temperatures well below ambient air are possible." - Natural Solar Architecture, a passive primer, David Wright (is there a convention for sighting things in this media?) The next page of the book also mentions Dew Ponds in England which can have similar effects under certain conditions when covered during the day.

The parabolic reflector seems odd to me, but I am not well scienced enough to know if it actually will reflect something to aid cooling.

/ The question, is the amount of radiation they emit a function of anything other than their temperature? /

[Vernon]'s IKECE idea bandies about these questions. Linked.

I suspect that the parabolic mirror would work better if it were black, since that would better absorb heat radiated down from the focal point object instead of reflecting it back onto the object. I agree with [ldischler] about the reflective insulated bucket, except I think it should be black, not reflective. Reflective outside would be good.

Aha, so Vernon is creating an electrostatically assisted radiator - very nifty.

Going back to the two objects in space described earlier(minus a Vernon-style amp-Radiator), they ought to be equally radiative in all directions. i.e. if you look at them from any angle, the IR illumination coming from them is going to be the same.

This time, the question is whether there is a way, by changing their shape (by forming some kind of parabolic radiator as described here)or by bolting on some self-contained device (as per Vernon's idea), that you can get the bodies to emit their radiation in a specific direction?

And if so, in the vacuum of space, mightn't this qualify as a form of propulsion?

Changed my - to a +. This is sort of like holding a parasol over your head on a sunny day. But this parasol is upside down shielding you from you hot earth glow. Would be interesting to see if the parabola shape is much more efficient than a bucket shape. And yes reflective to heat would be better than black, since you want that heat sent out and on its way, not heating up the reflector.

Hmmm so there is a difference between radiance and conductance (of heat)...

There is also a difference between using a parabolic mirror (without glass) and placing an object on a flat mirror (without glass) --- in the second scenario conductance is used to help cool the object sitting on it...

No doubt it works better though... (not because of the shape but because the mirror touches the object and the mirror is more conductant and radiant)

If the edges of the flat mirror where bent upwards (parabolic) then they would shield the object --- from any radiance except the night sky (which is dark at present).

Hmmm looks like [ldischler] made a mistake... [I would make the object touch the mirror though].

We have emperical evidence that the parabolic reflector /does/ work and allows water to be frozen on an otherwise non-freezing night. Read the first link in the section about cooling. Ice from water on a 46'F night.

The question is how to cool enough material to air condition the home or whatever all the next day. How much mass is reqired, what substance will be low cost and work well, how do we insulate that mass, how do we transfer the heat from the house to the mass?

The transfer of the heat from the mass into space is demonstratibly baked. The rest is what is still dough.

A simple way might be to have cooled water pumped from the "radiator" into storage barrels in the house just prior to sunrise.
At sunset the water from the barrels that have heated due to balancing with the house temperature can be pumped back into the radiator. Ha Ha. I forgot. This looks to be pretty much like what [BunsenHoneydew] suggested.

//We have emperical evidence that the parabolic reflector /does/ work and allows water to be frozen on an otherwise non-freezing night. Read the first link in the section about cooling. Ice from water on a 46'F night.//Which just shows you don't know the difference between a cone and a parabola.

Just realized - it's apparently a common feature of houes in North Africa to have tall, narrow courtyards walled on all sides specifically designed to catch cold air during the night and help cool the building during the day. Presumably they also take advantage of this effect.

Two things.
0) Yes, putting an object into a vacuum
flask and shading it from the ground
will cool it.
1) It doesn't make a difference whether
it's a parabola or a bucket. As long as
all of the reflection is coming from dark
sky, who cares whether it's a small, in-
focus patch of dark sky?
2) The device won't actually cool to 3K.
At optical frequencies, the object will
pick up power from starlight. I'd guess
that this would equilibrate out to a few
dozen K. Build your device in a crater
on the lunar North Pole and you'll get
this. On Earth, though, the atmosphere
itself is at 200K or so, and it's
opaque---and radiates in all
directions---in the far infrared. This
will limit the ultimate temperature of
the device.

That is not the point
- a bucket will do what it is meant to and hold the radiant heat (i.e. reflect it back to the object)...
- a parabola will do what it is meant to and direct the radiant heat upwards and outwards...

[
I am still not convinced it works in practice though --- will have to isolate the atmosphere --- ever seen that movie perfect storm ?

For example if you happened to point an infrared temperature sensor into a just opened freezer you will notice the temperature rapidly rise from -8 to 0 degrees C --- this is due to the convection of air.

Convection being the 3rd process driven by heat that applies to this idea

Umm was just thinking putting the thing in a vacuum would tend to decrease boiling temperatures... Oh well that is a different story.]

The conceptual mistake youre making is this: the insulated bucket (no matter what shape) and the object inside it will reach the same temperature. And if theyre at the same temperature, the geometry doesnt make any difference.

What you have created could be termed a "source-avoidance bolometer". Yes, the parabolic shape will help keep you from accidentally getting inputs from first magnitude stars, the full moon, passing jets, etc...

ldischler, you're thinking about
blackbodies in equilibrium. This system
is neither a blackbody nor in
equilibrium. You have a small, hot,
black object. Does it warm or cool?
This depends on the difference between
the total energy of radiation incident on
it, and the total energy of radiation
emitted from it. Emitted: thermal
spectrum at the body's own
temperature. Incident from above:
night-sky radiation. Incident from
below: night-sky radiation bouncing off
a mirror, plus thermal radiation from
the mirror itself. What's the thermal
radiation from a room-temperature
mirror? Much, much less than a
blackbody of the same temperature.
The mirrors of the Gemini telescope
have an emissivity of about 2%---that's
how they can use the telescope to see
mid-infrared light, otherwise they'd
never see anything but their own
mirror. As long as the night sky (plus
atmosphere, plus mirror) radiation
fluxes add up to less than the object's
blackbody flux, the object will cool.

//[this device] will attempt to bring the temperature of a body at its focal point into equilibrium with the cold vastness of space//

The dish cannot know *anything* to do with the temperature difference between the radiator and some coordinate out there somewhere in deep space. (assuming it to be far away - and in fact changing..)

It would constitute 'spooky action at a distance' - ie) in violation of special relativity.

Maybe.

It will still work however - as it seems though from the discussions here that in fact, the dish should be pointed somewhere with the lowest 'density' of radiation is coming from. (As not to _gain_ additional heat from a radiation source).

Is the Earth as a whole really losing more energy here than it would otherwise ? (I thought about this for a few minutes and 'yes' is what I came up with - because insulation can obviously slow-down heat loss; apologies again to more physics-savvy bakers...)

True enough, bm-gub, though it should make no practical difference to radiative transfer if the mirror is a tub or a parabolic. On the other hand, placing the object in a deep cone or well should work better as it would minimize convection.

Just noticed a slight and slightly amusing resemblance between Ludwig Boltzmann (Austrian physicist famous for his founding contributions in the fields of statistical mechanics and statistical thermodynamics) and TV's 'Cracker' portrayed by Robbie Coltrane.

Sorry if I'm repeating an argument (I've only been half-following the discussion). If we neglect convection*, wouldn't a parabolic dish be the best option simply because the hot object won't "see" itself from most directions? Take the tub example. Directly under the hot body heat is reflected back to itself. The same is true for a ring diagnally downward from the hot body, using the cone model.

*which I know we shouldn't, but without detailed design we can't debate the mirror shape's effect on that much

Wow. I'm loving the amount of intelligent annotation this one is kicking up. I'll attempt to answer and comment on some of the points raised.

[JamesNewton] //The thermal storage unit is the one that is most expensive, I think, because it must be both large and very well insulated.// A propane/LPG tank should do, with added insulation of your choice. Of course all refrigerant plumbing will need to be insulated as well.

[zen_tom] //but you could beam it at anything you want, and you should recieve the same levels of local cooling.// Not so. Whatever you beam it at will beam its own radiant heat straight back at you. Also [monojohnny] yes, it's just somewhere to aim that radiates back a significantly lower amount of energy than the object puts out.

[idischler] //block incoming photons while allowing outgoing photons to escape// Hmm. Any physicists want to comment on the possibility of an IR one way mirror?

Parabolic vs not: I wonder about the effect of focussing as much of the energy as possible into a small arc of the greenhouse-absorbing atmosphere, vs spreading it wide across the sky.

[Worldgineer] seems to have pointed out the real advantage, which didn't occur to me: //the hot object won't "see" itself from most directions// To take care of the small area at the base which reflects back to the tube, use a compound parabola [link] The first image on that page is the closest to the picture in my head [moomintroll]. And make the curve as steep as possible.

[idischler] //the body won't get any colder than the reflector, so that has to be well insulated. ... as a radiator, the set-up is inefficient// Darn good point that first one. The reflector will need to be well insulated, and perhaps the refrigerant can be run behind it to cool it as well. Whether that is best done before or after the central tube is a mystery to me right now.

Evacuating the space between the central tube and the reflector would help with conductive insulation.

Uh, isn't not being a source of photons the definition of cold? If you're distant from or shaded from a nice big hot sun, then it's pretty cold out there, no?

A cone is an imperfect parabola.

[zen_tom] //And if so, in the vacuum of space, mightn't this qualify as a form of propulsion?// Would you mind if I spin this off into another idea? I will credit your inspiration of course.

[Heathera] Brilliant summary!

[madness] //I would make the object touch the mirror though// Hmmmyesss. I could see how that would help. Use conduction to drag heat out of the pipe and radiate it away. That would also solve the problem of the reflector being a heat source.

Or just make the tube into a double walled parabola, silver and insulated on the outer side, and black on the inner.

[monojohnny] //frequency// The frequency is directly related to the absolute temperature. Unfortunately most of the frequencies we're dealing with here are opaque to the atmosphere's greenhouse gases. Hence my brainwave about flourescing these down to lower frequencies outside the greenhouse-opaque band. I -think- that should allow you to truly exploit the temp diff with space, and not just the night sky.

//Would you mind if I spin this off into another idea?// Spin away! I put it forward as a kind of 'if x then y' implication of directed radiation (which I'm still not sure I believe in)

If it is possible then there ought to be a shape(perhaps a cone, or hollow-cone-type shape) that, given a suitable heat source, is capable of self propulsion through heat emission, leading to the potentiality of objects occupying powered and otherwise impossible orbits around hot objects.

I think the shape of the reflector isn't a major factor in this. The reflector acts as a heat shield here, shielding your "hot body" from the IR radiation of the surroundings. That reduces the heat recieved by the hot body. The heat radiated by it to the reflector will either be reflected or re-radiated outward (to your cold sink) or re-radiated towards the hot body. The parabola will prevent reflection back to the hot body because of it's shape, but how much it affects heat loss depends on the reflectance/emissivity ratio for the reflector (in IR wavelengths). If the emissivity were very low and the reflectance very hign, it could become significant.

The "cold sink" - the night sky, because it's IR opaque, will act like a black body with a temperature above the cosmic background temp. Heat rejected upwards is going to be related to something like: {integral of dT/dz}^4*{integral of de/dz}, e being emissivity in IR, z being height from 0 to some arbitrary edge of atmosphere. So it'll be colder than ambient, but warmer than space. If you then had a perfect reflector (zero emissivity), it effectively doubles the heat exchange area, but you still can't get colder than your effective sink temperature.

/what we have is a hot body who's radiative energy is being directed away by our parabolic *reflector*./

I think a parabola is the wrong shape. Consider some water to be cooled which is radiating away its heat, "cold" space with no radiation, and warm Earth with much radiation. What object to put between water and earth to maximize cooling of the water? Let us assume zero convection, zero conduction.

1: The object should have maximum emissivity, which is just a function of reflectivity. So it is a mirror.

2: The object should block radiation from the Earth to the water. Many things would suffice - a bucket, a big flat sheet, etc.

3: The object should minimize reflection of heat emitted from the water back to the water. A parabolic mirror is the wrong shape for this. It is designed to collect incoming radiation and concentrate it back to a point. In the linked experiments where they made ice, placing the water to be cooled at the focal point would maximize its exposure to radiation emitted from the water itself and reflected off the dish.

I think a cone would work better. I have linked up my first HB sketch. As the water emits radiation, only that which strikes the tip of the cone will come back to the object. All other radiation from the water will be reflected away.

As regards nonreflected "blackbody" radiation from the object, I do not think shape matters since each point on the surface will radiate heat equally in all directions.

Ah, but your cone won't block radiation form surroundings unless you increase the angle to the point of being a flat surface. Assuming your hot body is small compared to the reflecting surface, the hot body will only reflect back to itself at one point using a parabola. I'll link to a picture of that.

I could imagine a small cone at the base of the parabola to account for the hot body not actually being a dimensionless point.

It depends how far the water to be cooled is from the earth. In a space situation (hot body, reflector and sun/planet) it would be possible to accomplish this using a cone because the base of the cone need only be large enough to eclipse the sun/planet in question.

With a backyard type setup, you are right because the hot body (the water) is too close to the earth. To eclipse the earth, the base of the cone must be impracticably large.

A friend made a 2' cube out of 4" thick blue styrofoam with a hinged front door and afixed to the cieling of the interior a "several" inch deep stainless steel pan filled with water - interior heat sink.

He pulled the center of a 50' coil of copper tubing upward to form a spiral cone "christmas tree" and conected the top and bottom of the tree with a vertical coper tube and either filled it with water or made it into a spiraled heat pipe (don't remember).

He installed it with the larger bottom most coils in the pan of water,the top smaller radius portions of the coil being above, outside of, the top surface of the cube.

He put it in the desert, put his beer inside, closed the door and the beer got cold.

This is getting to the point where we need some half-baked experiments to get real half-baked results. Go find some buckets, spray foam, tinfoil, thermometers and at least three beers. The control beer rests on the ground overnight, the others get trapped inside various contraptions in the hopes that their heat will be sucked into space. Go on, do it, I dare you.

Good call [Heathera]. I think we need a third set of beer cans, so we can test for parabolic vs any-shape reflectors. And maybe a fourth, so we can compare with night-air cooling, like [lumbergoofie] friend's system, and the system [baconbrain] linked.

I think I agree with you about straight up along the shortest path through the atmosphere being optimal, but I'm fuzzy on the logic there and willing to be proved wrong.

A true parabola is the right shape to prevent self-reflection only when dealing with a theoretical point source. As the pipe has significant width, we would need to use a compound parabola. If you look at the first image in the "compound parabola" link above, you'll see it combines [Worldgineer's] //small cone at the base of the parabola// with parabolic curves either side.

When I first read this idea, I thought "no, that can't be right", but thanks to all the discussion, particularly [heathera]'s "coldest, thinnest slice of atmosphere", I'm convinced - a compound parabolic reflector could achieve really low temperatures on a clear night.

I love this place, I'm learning all the time. "Steiner for mad engineers" - brilliant.

"A parabolic reflector pointed at a clear
night sky will tend to bring the
temperature of a body at its focal point
into equilibrium with the cold vastness
of space, at approximately 3 kelvin."

Only if:
* you first remove the atmosphere of
the earth.
* The parabolic reflector is a perfect
insulator.

[MFD bad science removed. I don't feel
like it's bad as much as
misunderstanding the principles
involved. The parabolic shape works
well to cool something at its focus
because none of the heat radiated by
that object will be reflected back at it. If
you want to cool a large thing like the
roof of a house you'd be better off
letting the whole thing radiate normally
rather than letting only a small area
radiate slightly more efficiently. The
cooling below ambient that has been
observed is because you are changing
the environment, not surprising that
you see achange the environmental (or
ambient) temperature.]

[-] For using already baked (and quite
nifty) 'parabolic oven as freezer'
concept to actually reduce the cooling
efficiency of a flat roof.

No, it /isn't/ bad science, it should /not/ be mfd and if you read the discussion before commenting, you should see that the effects of the atmosphere and insulation or lack there of have been integrated. It just blows me away that people can't accept that the use of the parabolic reflector /has been demonstrated to work!/ I personally have seen this effect when demonstrating dew collection for survival classes years ago. The water in the bottom of the plastic lined bucket we used to collect dew became ice covered on a night that never got much below 70'F. I don't have the exact temperatures or details, but in retrospect, that is what was happening. And the very first link clearly shows that it does, in fact, work.

[st3f], I agree that the atmosphere is a problem, but I wouldn't be so confident about calling this bad science. Note that a "time" factor is not really mentioned, and it is important, too. The dark side of the Moon, for example, has two weeks to radiate into space before it sees the Sun again, and even with essentially no atmosphere its temperature doesn't reach 3 Kelvin. This is because of the "thermal mass" of the radiating landscape; it simply takes more time for that thermal energy to be radiated away than two weeks --and when the Sun shines on that landscape again, it re-acquires as much thermal energy as it lost.

So, an object at the focal point of the proposed reflector here has thermal mass, too. It will need to be mounted in a vacuum, so that convection/conduction of atmosphere doesn't bring heat to it as fast as it radiates away. Also, the parabolic mirror will need to be either SERIOUSLY parabolic, or located at the end of a tube, like a telescope. The object at the focal point needs to be shielded from radiant energy arriving from all around.

I think that if the other end of the tube was the glass vacuum seal, and if there was a light-proof cover that could be employed in the daytime, AND if the Object-to-be cooled wasn't part of a cooling system (where heat is added to it, for radiating), then enough night-time exposure of the Object, possibly months, could get its temperature most of the way to 3 degrees Kelvin. Do remember that the ancient Romans, in the North African desert, made "ice wine" by digging a pit and putting low-alcohol beverage into it, and using straw and such in the daytime to shield it from the Sun. After a few nights, the water got cold enough to partly freeze, yielding ice that the Romans simply picked off and threw away, and the percentage of alcohol to the remaining water in the beverage went up significantly.

So I'd say the only fault with this Idea is that it is unrealistic about the time factor, needed for significant cooling. Not what you said.

Where did you get that about "ice wine"? All I can find refers to making wine from frozen grapes.//could get its temperature most of the way to 3 degrees Kelvin//Doesn't matter how long you leave it, you're not getting down to 3 degrees. The equivalent sky temp is higher than that. Much higher. Think about it. At 3 degrees K, oxygen and nitrogen would freeze solid. Not bloody likely!

I like that Roman Ice wine maneuver. Sort of the opposite of distilling: instead of driving the alcohol off, collecting it and adding it to flavored water, drive the water off and keep what is left. The stuff left might be very yeasty, but it is all nutritous.

Neglecting conduction & convection is a huge suspension of disbelief, but that's ok on the HB.[+]

Radiant heat goes as fast as the difference between the squares of the absolute temperatures. Conduction/convection often go linearly with the diff only (not diff of squares). So, radiancy only dominates at really high delta T's. It's highly unlikely to have enough insulation to do this on earth.

It could work to cool ships in space, but there is little need for that, as they are already radiating from every surface into 3 K.

Of course, all of the above is of course nothing new to 1/2 the anno'ers and the author.

I think it was clear early on that you couldn't get down to 3K because of the atmosphere, but the principle is still good, the idea valid - and probably worth implementing.

Making a rough calculation is easier if ignore factors like conduction and convection. They are difficult to calculate without making assumptions about size, shape and temperature of the hot body, the materials, and the surroundings.

When you came to design a real system, you'd take measures to minimise conduction and convection. But in this case, to work out "will it work or not", it makes sense to ignore them.

Again, making simplifications about the reflector allow a "will it work" kind of assessment. In practice, it doesn't have to be a perfect insulator (or a perfect reflector) - the better it is at insulating (and reflecting), the better the performance of the cooler.

But it would *still* work, with real, every-day materials, to achieve real, practical cooling.

For me, that's a shock. When I first saw the idea I was totally sceptical, for all the reasons discussed.

[ldischler], "ice wine" was something I read about decades ago, before the Internet. I am surprised, though, that you couldn't find a reference. Next, regarding what I actually wrote, and what you quoted, instead of what you talked about, "could get its temperature most of the way to 3 degrees Kelvin", do note I left at least one hedge in that. If ordinary temperature is about 300Kelvin, then "most of the way to 3K only needs to be 51% of 300-3.... :) I agree that there is a problem with radiant energy from the atmosphere coming down the parabola/tube, and I agree that you can't reach 3K because of it. Still, the column of air above the parabola/tube has a small target to radiate into; it mostly radiates in all directions...so I think some significant degree of cooling is possible.

Perhaps a parabola at the bottom coupled with a tower of some excellent insulating material. The tower would limit the mass of air that could radiate into the ice and get you closer to your 3'K. This would also allow many units to be placed right next to each other and allow placement right next to buildings, trees or other objects that would otherwise act as heat sources.

I know from experience that if you lie at the bottom of a tall enough smoke stack, close up the opening so it is dark inside and wait for your eyes to adjust, you can see stars in the middle of the day. I wonder if that same effect would allow our water to freeze duing the day... Assuming the stack was not pointed at the sun! I remember it being darn cold in that furnace, but I think that was just the earth around it.

sophocles is not entirely right. The atmosphere is generally opaque to long wave radiation, but there are many windows, so the effective sky temperature can be 10 to 15 degrees C cooler than the surface air. (Obviously it has to be cooler, or the earth would never lose any heat.)

For your air conditioning use you would want to use a parabola which was constructed using heat conductive metal such as aluminum. You would run your coolant through the entire parabola probably as rings of tubes on the back side. In this configuration the entire dish becomes your radiating surface while the parabola also provides the shielding walls. You would coat the dish with high emissive white paint. The cooling of your system in this configuration would be 50 to 80 watts per square meter depending on ambient conditions. If your fluid is substantially warmer than ambient the rate of emission goes up. Efficiency will be higher if you can shield it from convective heating, wind and dew which is particularly important when your fluid temperature drops below ambient. I tend to think the additional cost of fabricating the parabola would be enough to encourage use of easier to install flat panels. Whenever your ambient temperatures are below your coolant temperatures you will use a standard convective radiator (Like and automobile) to bring the fluid down to ambient. Once the fluid reaches ambient then the convective radiators would be removed from the loop to continue cooling below ambient.

The experiment by the BYU professor Parabolic oven as freezer would seem to confirm your hypothesis since they obtained 20F differentials. Upon a more extensive review of literature it appears that the same 20F differential has been obtained using simple flat panel radiators.

A study funded by EPA which indicates that rate of transmission of black body radiation is limited by the size of the object and that its rate of transfer can not be increased by any combination of mirrors or reflectors. All other references I have been able to find are also negative on that parabola or cone having the effect you expect.

I suspect that what happened in the BYU test is the reflective foil prevented infrared in the local environment from reaching and re-warming their water while the reflector also ensured that most long wave (blackbody) energy shed by the object was reflected away. The double polyethylene bags minimized local convective heating. It appears the funnel wasnt doing anything significant that a broad flat surface and reflective sides would not also have done. It did provide a deep pooling area for cold air which helps prevent wind re-warming. The funnel shown would create a condition where some of the long waves radiated from the sides of the object would actually bounce back into the object causing it to cool at a slower rate than a simple flat radiator. The guys at BYU are pretty bright and I am hoping that my analysis is wrong.

According to the literature the theoretical maximum for long wave radiation heat loss in dry air is 110 watts per square meter of radiator surface although one patent claims 150. Under many conditions 70 to 80 watts sq meter is the max in dry air and 50 to 60 in humid air with the system not working at all when covered by thick clouds which emit about as much as they absorb

The amount radiated is tied directly to the surface area of the radiator, its emmisivity and it's temperature. Your idea of using the parabola to radiate for a smaller object in the center will not work because the blackbody energy radiated is limited to the surface area of the object in the center. The object in the center may get cold but it will not move the amount of energy you are hoping for. You could heat the entire parabola and it would provide roughly the same functionality as an equivalent surface area of flat black plate. The interesting thing about the parabola is that it pretty much prevents long heat waves (blackbody) from other objects especially those below and to the sides from interfering with the process by re-warming the cooled object.

The literature indicates average cooling tends to be 5F to 7F below ambient and a few mentioned 20F. It appears that convective heating and dew point tend to be the key limiters. As I mentioned below a deep parabola would help minimize convective warming by creating a stagnant air zone where cold air could pool. I would be interested in the maximum real cooling achieved by your tests.

Do you have any links to your original science for the using a parabolic mirror to radiate away blackbody energy from the point of focus?

What a parabola would to a particularly good job of is ensuring that no other objects emitting long waves are unable to reach the cooled object from the sides or bottom. It would also ensure that any long waves emitted from the sides of the object are bounced away into space rather than bouncing back into the object. Depending on the depth or aim point it would also block long waves from shallow angles (closer to horizon) from reaching the object in the center. It may appear that the parabola is actually focusing the heat way but heat radiated away is still limited by the sum of the exposed surfaces of the cooled object in the center. In addition depending on its emissivity the parabola surface would cool rapidly due to the same factors and as a result cold air tend to pool in the bottom of the parabola which provides a nice cold stagnant insulating layer surrounding the object in the center which helps reduce convective heating of that object. The aerodynamic shape of the parabola can help prevent wind from stripping the stagnant insulating layer away which can improve overall thermal performance under moderate wind.

Air is relatively transparent to long wave radiation so while these waves will have moderate absorptive warming of local air most of this type of heat energy will travel a considerable distance. The further the energy can travel before it is absorbed the higher the likely hood that whatever absorbed it will miss your object when it is re-emitted. Humid air absorbs and emits more long wave energy which is one reason efficiency goes down in humid environments.

I think a close to ideal shape would be a broad warm flat radiating plate that is surrounded by a fairly deep parabola designed for a focus point at least 3X the width of the radiator plate high. The deeper the parabolic sides can be made the better they will work to prevent convective heating and wind losses but it may be cheaper to use a thin polyethylene sheet stretched across the top of the walls. The bottom of the radiator would obviously need to be shielded protected from picking up heat from the ground or roof. I would go with high emissive white paint to minimize warming from the sun.

One final point is that on a clear night the amount of energy received from direct overhead is so little that it can be considered statistical noise when compared to convective, wind and dew heating. The real question is how much energy is being shed versus how much energy is being absorbed. The night sky doesn't emit much heat so it is an obvious place to aim the radiator. What is really happening is that a majority of heat (long wave radiation) is being absorbed by the air before it ever reaches space but it will have traveled a considerable distance before it is absorbed. When the heat is is reemitted it tends to go in all directions so very little is even remotely aimed at your cooled object and the further it goes before being absorbed the harder it is to aim at a relatively small object.

You have obviously been looking at this longer than I have so I would be interested in your opinion.

NOTE: Convective heat exchange is good as along as your radiating surface temperature is above ambient. It is actually much more efficient to let a brisk wind carry heat away than it is to radiate it away.

Hot air rises. The hot stuff at the top of the atmosphere will both (1. expand and thus cool. 2. exchange heat with space).

The experiment didn't seem too rigorous. A little bit of moisture evaporating off can explain much cooling in dry desert climates.

A more rigorous test would be to have units side-by-side, one with "blackbody radiative" surfaces, and the other, identical, but with lower emissivity surfaces. Compare to get the differences that are strictly due to radiation to space (or atmosphere).

[joeatxdobs] //You have
obviously been looking at this
longer than I have// Uhh, from
the -incredibly- detailed analysis
in your anno (thanks), I'm not
sure that's
really true. The basic principle
came to me in a flash and the
exposition of the detailed
workings came as I wrote. The
annos here (positive and
negative) have helped me
enormously in working through
this and halfbaking workarounds
for some of the objections.

Of course I was way over the
top in talking about liquid
nitrogen type temperatures.

Re your point about costs vs flat
panels: I should clarify that I'm
thinking of arrays of small linear
parabolic troughs in a sheet
arrangement, rather than a single
full 3-D parabola. The elements
of the linear system could be
roll-formed or stamped,
assembled in continuous line
processes and cut to the
required length; connectors and
end seals would then be added.

I like your suggestion about using
an auxiliary radiator to equalise
with ambient. On the other
hand, I could see that the
existing pipe network I describe
above could be used as that
ambient air radiator, by opening
and closing the transparent cover
over each trough.

An engineer recently mentioned
to me that there is a size
threshhold for air gaps, below
which thermal induced air
circulation is suppressed (ie it
stays still). Still air is an
effective insulator, apparently.

So, a sensor compares fluid
temperature with ambient air,
and pops open the lid of each
channel when the air is colder
than the fluid. The pooling
effect you mentioned helps
here. With the lid open, air can
move freely through the system
and conduct away heat.

When the fluid approaches
ambient, the lid pops down
again, trapping the cold air in a
below-convection-threshhold
sized space. The still air won't
be as effective an insulator as
vacuum, but a hell of a lot
cheaper to make. The system
then operates as a night-sky IR
radiator as originally discussed.

I picture the lid being a single
piece structure for each sheet in
a modular trough system, with a
single open/close mechanism to
reduce cost and complexity. To
reduce wind loading I imagine it
would pop straight out a few
inches but stay parallel to the
surface, rather than hinging up
like a door.

Just out of interest [joe] what's
your background? Are you in
engineering or HVAC/refrig
work?

[sophocles] //Hot air rises. The
hot stuff at the top of the
atmosphere will both (1. expand
and thus cool. 2. exchange heat
with space).// Sure, but it will
also be replaced pretty quickly
with air from below and
beside.

//YOU ARE RADIATING
TO THE AIR RIGHT ABOVE YOU
(not outer space)// Yes, but
only to the IR opaque portion of
the atmosphere, ie the
greenhouse gases. I admit
this is a more significant factor
than I first realised, hence my
burblings about magic IR one way
mirrors and fluorophores.

//Also, you're talking about a
parabola with a focal point of
outer-space?// No. The focal
point is the radiator tube in the
centre. Radiation from this
point comes out of the system all
moving in parallel straight lines
upwards.

In all the above, "parabola" is
really shorthand for
"mathematically optimum shape
which I suspect is a compound
parabola but modelling or
realworld tests may prove
otherwise". If the reflector
becomes the fluid carrier and
there is no central object to
focus on, then other shapes may
be optimum. In a design for
real-world manufacture and
installation, cost and reliability
are definitely optimising factors.

For example:

[James Newton] // a parabola at
the bottom coupled with a
tower of some excellent
insulating material // Here the
problem would be preventing
the inside of the tower itself
radiating back into the system.
The theoretical optimum
parabola/chimney combination is
a single, very steep and deep
parabola so no re-reflection
occurs. And the higher the
tower/parabola, the more
expensive, the more insulation
required, etc etc. [joeatxdobs]
your height of 3x plate width
sounds reasonable. Is that a
guess, an educated guess, or did
you do some figurin'?

//at the bottom of a tall
enough smoke stack ... you can
see stars in the middle
of the day. I wonder if that
same effect would allow our
water to freeze duing the
day... // I suspect it may, given
enough time. There's a huge
abandoned smokestack just
around the corner from my
house as a matter of fact. I'm
gonna do me some stargazing.

Instead of inventing a completely new method of refrigeration, why not combine this with more conventional refrigeration methods to increase efficiency. Replace the conventional condenser coil, which relies on convection and conduction, with a cosmic radiator, and the efficiency should be increased since the effective sky temperature is lower than ambient air temperature (at night,) and it wouldn't require forced air. Also, I think the necessary surface area of the radiator would be reduced since the temperature of the refrigerant would be elevated by compression. Combined with a storage scheme, you could have efficient cooling utilizing off-peak electricity.

Alternately, if an absorbtion chiller is used, the radiator could be used to absorb energy from the sun during the day. The sun provides heat for the absorbtion chiller while a convection/conduction type exchange is used for cooling water. At night the process is reversed, with the cosmic radiator providing the cooling water and the convection/conduction exchange providing heat from off-peak electricity.

Or... another possible absorbtion scheme is to store the suns heat during the day, and then use the stored heat at night, while the radiator is providing cooling water and the resulting chilled water/refrigerant is stored for use during the following day.

[bm-gub] //Incident from below: night-sky radiation bouncing off a mirror//

Ah hell. I'm trying to imagine a way of eliminating this through geometry or materials but so far all I'm getting is a headache. Perhaps this is where the IR fluorophore comes in...

[Heathera] //coldest thinnest slice of the atmosphere - straight up./// I'm less and less sure that matters, as it appears from [ldischler]'s correct-sounding argument that it's the temperature of the target that matters, not the heat capacity. The bit of atmosphere at 200K you're aiming at is continuously being refreshed by new bits of atmosphere at the same temperature. But trying to think clearly about that is adding to the headache.

Wow Bunsen this is about the same conversation as Mercury 2.0 turned into, except without the atmosphere. I thought you took off on my idea until I read the dates. Good conversation, I think I have it all figured out but this probably doesn't need to be commented on other than I am impressed by the thought process, and I am jealous of all the buns.

Space is cold because it's a vacuum, and therefore light doesn't get absorbed by it. The background radiation does not cool space down, but it actually heats it up to 3K, and without this cosmic background radiation space would be at 0K (also, without all the rest of the radiation too). All that you are doing when you focus this microwave radiation at a point is increasing the flux of the poynting vector at that point---more watts per square meter times area---i.e. heating that point up.

I saw something like this in an old Mother Earth News - the guy had built his refrigerator from scratch, and had coolant lines that went to a big heat exchanger on his roof. Yes, he lived in the American Southwest, so cool dry nights were the norm.

The refrigerant dumped a lot of heat to the sky during the night, and just sat in the reservoir in the chilled space during the day. There may have been a valve in there somewhere, I forget. The neatest part was how his total electrical load for the fridge was 1/2 kwh per year, to run the bulb.

It won't work here in TN. Muggy nights make the sky itself a heat source.

[quantum_flux] - //All that you are doing when you focus this microwave radiation at a point is increasing the flux of the poynting vector at that point---more watts per square meter times area---i.e. heating that point up.//

Yes, heating that point up - but heating it from a 3K* source, while it re-radiates at closer to 300K. I can live with that.

What interests me about your anno is this: is it better** to gather the CBR from as small as possible an area of sky, as large as possible, or it makes no difference?

The first suggest a parabolic mirror, and the third just any old shape.

* alas, here under that pesky atmosphere, we only have access to a ~200K source. Still the point is the same

** "better" meaning fewer watts at the focus.

I like the pun of "poynting vectors" but I have no idea what they are.

[elhigh] - yes, I'm picturing something like this pre-packaged in a shipping container for remote communities in the Australian outback. Should save a fortune in diesel.

// the theoretical maximum for / heat loss in dry air is 110 watts per square meter / although one patent claims 150. Under many conditions 70 to 80 watts sq meter is the max in dry air and 50 to 60 in humid air //

Which makes this a heck-load more efficient than a solar-panel powered conventional refrigeration plant, both in terms of area and cost of materials.

// energy radiated is limited to the surface area of the object in the center //

Take the original circular cross-section tube and compound parabolic cross-section reflector. Extend the tube in the centre horizontally to the greatest structurally sound width possible, making it a hollow flat plate. Extend the cone-shaped bit of the reflector underneath to suit. You end up with a double-sided flat plate emitter, one side radiating straight up, and the other side radiating via the compound mirror.

I think the concepts behind this have been baked for centuries. The old ice houses in Iran (Shiraz and Esfahan) used to use night sky radiation to make sheet ice in winter and then store them in large subterranian structures to last through summer. The skytherm house in Atascadero, CA by Harold Hay is another use of this idea.

A parabolic dish wouldn't be the optimal shape., preferably one would use a parabolic trough identical to the ones used for solar thermal. as mentioned you're trying to get the maximum amount of isolated surface area focused on the emptiest portion of sky, you don't need point focus so much as a linear focus. also due to angle of refraction it might be advisable to verify if pointing straight up is the optimum angle.

finally to limit convection and conduction, the obvious answer is to build the devise in an evacuated box. This is identical to the boxes used for radiant solar heating, that have evacuated tubes or the whole box is evacuated. Which makes me wonder, what's stopping a radiant solar heater from being purposed into a solar cooler at night? or what prevents them from acting as one?

Also a similar concept instead of using the sky as a heat sink, you can use the ground which always stays around 60 degs year round below a certain depth. it's called ground source heat pumping and is very baked

I suspect that even if one could come up with the cleverest parabolic shape reflector design possible, insulated everything to the nth degree and then pointed it to the coldest part of the universe you wouldn't cool anything if you submerged the whole apparatus in a swimming pool of warm water. Think of the atmosphere as the warm, conductive medium and you have the same outcome.

Parabolic troughs were mentioned in the original post, and several times in the annos. They were always my intended form, not a circular cross-section dish.

Compound parabolas account for the fact that the central pipe is not a line of zero width. See [link 2]

// This is identical to the boxes used for radiant solar heating / what's stopping a radiant solar heater from being purposed into a solar cooler at night? //

That ... is an excellent point.

Using [joeatxdobs]'s figures for watts per square meter above, and the dimensions of a standard 20 foot shipping container, I get a total cooling capacity of about a kilowatt. Which is, admittedly, disappointing, but if the materials were cheap enough, could be a useful supplement to conventional refrigeration.

[AusCan531] Yes, the existence of the atmosphere - blocking some, but not all, frequencies of longwave radiation - has been extensively discussed in the annos. The conclusion was that the night sky is a ~200K radiative source/sink, not a 3K source/sink. Still useful. Various hypothetical workarounds have been proposed - IR fluorophores, one-way IR mirrors.

It should be trivial to build a shallow pool with an automatic cover that goes on when the sun comes out or on cloudy days and comes off at night. Then run water pipes through it and you'll have your 60 watts of cooling per square meter, no?